Apparatus for synthetic diamond deposition including curved filaments and substrate cooling means

ABSTRACT

Diamond is deposited by chemical vapor deposition on two parallel substrates, by means of a plurality of filaments between the substrates. The substrates and filaments are in vertical configuration and the filaments are prestressed to curve in a single plane parallel to the substrates, to allow for thermal expansion and expansion caused by filament carburization. The apparatus includes at least one and preferably two heat sinks to maintain substrate temperatures in the range of 900°-1000° C., for optimum rate of diamond deposition.

This invention relates to the chemical vapor deposition of diamond, andmore particularly to an apparatus for use in such deposition.

Various methods are known for the synthetic production of diamond. Inparticular, the deposition of diamond coatings on substrates to producecutting and abrasive tools is known.

One class of methods developed in recent years for synthetic diamonddeposition consists of the chemical vapor deposition (hereinaftersometimes "CVD") methods. For a general summary of various diamonddeposition methods including CVD methods, reference is made to Chemical& Engineering News, 67(20), 24-39 (May 15, 1989), incorporated herein byreference.

In the CVD methods, a mixture of hydrogen and a hydrocarbon gas such asmethane is thermally activated and passed into contact with a substrate.The hydrogen gas is converted to atomic hydrogen which reacts with thehydrocarbon to form elemental carbon, which deposits on the substrate inthe form of diamond. Many of the CVD diamond coating methods,hereinafter referred to as "filament" methods, employ one or moreresistance heating units including heated wires or filaments, typicallyat temperatures of at least 2000° C., to provide the high activationtemperatures at which these conversions take place.

Various problems have been encountered in filament methods of CVDdiamond deposition, and they have inhibited to a considerable extent theusefulness thereof on a commercial scale. For example, it is difficultto create conditions under which the deposition rate of diamond is highenough to be commercially feasible. Numerous methods employing ahorizontal configuration of the substrate(s) and filaments, withhelically wound filaments, have been disclosed, but for the most partthe deposition rate afforded thereby is low.

Also, the high substrate temperature usually produced is incompatiblewith a high rate of diamond deposition. At the filament temperatureswhich produce atomic hydrogen in the necessary proportions, thesubstrate reaches a temperature higher than about 1000° C. Optimumsubstrate temperatures for diamond deposition are in the range of900°-1000° C., and thus the substrate temperatures generated are toohigh for rapid deposition.

Finally, numerous problems with the filaments have been observed.Substantial expansion thereof occurs at the aforementioned hightemperatures; this is, in part, thermal expansion but to a greaterextent, in the case of the tungsten filaments frequently employed, isthe result of carburization, forming tungsten carbide with concomitantexpansion to the extent of about 20%. If such expansion is uncontrolled,filament breakage and/or contact between the filaments and substrate canoccur, either of which will terminate diamond deposition.

The damage resulting from carburization is particularly severe when ahelical filament configuration is employed. Carburization is accompaniedby cracking of the tungsten carbide, often in a spiral configurationwhich can cause deformation of a helically wound filament inunpredictable directions. The deposition operation is thus usuallyprematurely aborted, much short of the 30-40 day period normallyrequired for the deposition of a diamond film of 0.5-1.0 mm. inthickness.

The present invention is based on several discoveries of conditionswhich promote optimum diamond deposition on substrates. In the firstplace, it has been found that nucleation and growth of diamond ismaximized when the reactor configuration includes two substantiallyparallel substrates on opposite sides of a plurality of filaments,rather than substrates horizontally configured, or vertically in asquare or cylindrical array or one substrate between banks of filaments.In the second place, long filament life is attained with linearfilaments curving in a single plane to accommodate thermal expansion andexpansion caused by carburization. In the third place, employment of atleast one heat sink to regulate substrate temperature permits control ofthe deposition reaction to optimize diamond growth rates.

The invention provides an apparatus for improved production of CVDdiamond coatings by the filament method. The features of the inventionpermit closer control of filament configuration than has previously beenpossible, as well as control of substrate temperature for maximizationof deposition rate. One result is an increase in filament life, whichenables the deposition process to be continued until a coating ofsubstantial thickness is produced.

Accordingly, the invention is directed to apparatus for deposition ofdiamond on substrates by chemical vapor deposition, comprising:

a closed reaction chamber having at least one gas inlet and at least oneexhaust means, said chamber being capable of being maintained at apressure below atmospheric;

support means for supporting said substrates in said chamber parallel toeach other and spaced apart to permit gas flow between said substrates;

resistance heating means comprising a plurality of vertically extendinglinear, electrically conductive filaments prestressed to curve in asingle plane substantially equidistant from and parallel to saidsubstrates, each of said filaments being fixedly secured at each end toa pair of fixed electrodes; and

substrate cooling means situated adjacent one of said substrates on theopposite side from said filaments.

The invention will be described in detail with reference to thedrawings, in which:

FIG. 1 is a schematic side view of an illustrative apparatus accordingto the present invention;

FIG. 2 is a cutaway view of said apparatus on the line 2--2 of FIG. 1;and

FIG. 3 is a further cutaway view on the line 3--3 of FIG. 2.

Referring now to the drawings, there are depicted the interior featuresof a CVD diamond deposition unit according to the present invention. Allof said features are enclosed in a reaction chamber (not shown) which isair-tight and thus capable of being maintained at reduced pressure andis fitted with a suitable gas inlet and an exhaust port. All portions ofthe apparatus which are present in the reaction chamber are constructedof suitable heat-resistant materials, as necessary to withstand filamenttemperatures on the order of about 2000° C. and substrate temperaturesup to about 1000° C. Quartz is an illustrative non-conductiveheat-resistant material suitable for this purpose.

The features of the apparatus and associated articles which are shown inthe drawings include a pair of substrates 1, which are normally planaralthough they may be gently curved. Any substrate material suitable fordiamond deposition thereon may be employed; examples of such materialsare boron, boron nitride, platinum, graphite, molybdenum, copper,aluminum nitride, silver, iron, nickel, silicon, alumina and silica, aswell as combinations thereof. Metallic molybdenum substrates areparticularly suitable under many conditions and are often preferred.Supports 2 serve as support means for holding substrates 1 in positionparallel to each other and at a suitable spacing for deposition to takeplace.

The apparatus also contains resistance heating means comprising twoelectrodes and a number of vertically extending linear, electricallyconductive filaments or wires (hereinafter generically designated"filaments"), and otherwise being of conventional design and circuitry.The material of which said filaments are comprised is not critical, anymaterial known in the art as suitable for this purpose being acceptable.Illustrative materials are metallic tungsten, tantalum, molybdenum andrhenium; because of its relatively low cost and particular suitability,tungsten is often preferred. Filament diameters of about 0.2-1.0 mm. aretypical, with about 0.8 mm. frequently being preferred.

The filaments are located between said substrates, parallel to andsubstantially equidistant therefrom. Distances from filaments tosubstrates are generally on the order of 5-10 mm.

In the drawings, fixed electrode 4 is grounded and is fixedly attachedto a number of said filaments, one of which is designated 5. Since aplurality of filaments and associated structure are present, referencethereto hereinafter and in the drawings will be to only one; it shouldbe understood that the total number thereof is not critical to theinvention.

Insulator 6 separates electrode 4 and its base from conducting element7. The latter is conductively connected to a second fixed electrode 9,fixedly attached to the other end of filaments.

Each of filaments 5 is prestressed to curve in a single planesubstantially equidistant from and parallel to substrates 1.Prestressing of all filaments may be conveniently achieved by use of asuitable jig of conventional construction, configured to hold bothelectrodes and the attached filaments with prestressing beingaccomplished by pegs. Once the filaments are suitably prestressed, theentire assembly may be removed from the jig and mounted in the CVDapparatus.

It is highly desirable to maintain substrates 1 at temperatures in therange of about 900-1000° C., since within this range minimum reactionoccurs between the hydrogen present in the gas mixture and the elementalcarbon formed from the hydrocarbon therein; thus, said elemental carbonremains available to deposit as diamond at a high growth rate on thesubstrate. Absent any provisions for independently controlling substratetemperature, said temperature frequently exceeds 1000° C. and thediamond growth rate decreases substantially.

According to the invention, the desired temperature control is achievedby substrate cooling means comprising at least one and preferably twoheat sinks 18. Each heat sink is typically made of metallic copper andcooled by attached serpentine tubing 21 (also usually of copper) fittedwith cooling water inlet and outlet 19 and 20, respectively. Thedistance of heat sink 18 from substrate 1 is adjusted by a conventionalscrew mechanism controlled by crank 22, and said distance and the flowrate of water through the tubing are adjusted, either manually orautomatically via suitable sensors, to maintain the substrate within thedesired temperature range.

In operation, the reaction chamber of the apparatus of this invention ismaintained at a pressure up to about 760 torr, typically on the order of10 torr. A mixture of hydrogen and a hydrocarbon, most often methane andgenerally present in an amount up to about 2% by weight based on totalgases, is passed into the chamber and a current is passed through theelectrodes and filaments to heat the filaments to a temperature of atleast about 2000° C. With the substrate configuration employed, gasdiffusion between the substrates and in contact with the filamentspromotes excellent nucleation and growth of diamond particles.

The heat sink(s) is maintained at a distance from the substrate andwater passage through the tubing associated therewith is maintained at arate to provide a substrate temperature in the range of about 900°-1000°C., most often about 950° C. At such temperatures, diamond growth rateapproaches its highest value.

During the CVD operation, filaments 5 undergo thermal expansion andexpansion due to carburization. By reason of their prestressedcondition, however, such expansion merely causes them to bend further inthe direction of prestressing. Thus, they are not subject to failure asa result of contacting each other or the substrates. Using thiscombination of elements, it is possible to grow diamond films withthicknesses up to about 1 mm., or even greater on occasion, within atime span of 30-40 days without filament breakage or other untowardevents occurring.

What is claimed is:
 1. Apparatus for deposition of diamond on substratesby chemical vapor deposition, comprising:a closed reaction chamberhaving at least one gas inlet and at least one exhaust means, saidchamber being capable of being maintained at a pressure belowatmospheric; support means for supporting said substrates in saidchamber parallel to each other and spaced apart to permit gas flowbetween said substrates; resistance heating means comprising a pluralityof vertically extending linear, electrically conductive filamentsprestressed to curve in a single plane substantially equidistant fromand parallel to said substrates, each of said filaments being fixedlysecured at each end to a pair of fixed electrodes; and substrate coolingmeans situated adjacent one of said substrates on the opposite side fromsaid filaments.
 2. Apparatus according to claim 1 including a pair ofsubstrate cooling means on the opposite side from said filaments of eachof said substrates.
 3. Apparatus according to claim 2 wherein thefilaments comprise metallic tungsten.
 4. Apparatus according to claim 3wherein the substrate cooling means comprise metallic copper withprovision for passage of cooling water.
 5. Apparatus according to claim3 further comprising means for adjusting the distance from thesubstrates to the substrate cooling means.
 6. Apparatus for depositionof diamond on a pair of planar vertical substrates by chemical vapordeposition, comprising:a closed reaction chamber having at least one gasinlet and at least one exhaust means, said chamber being capable ofbeing maintained at a pressure below atmospheric; support means forsupporting said substrates in said chamber, parallel to each other andspaced apart to permit gas flow between said substrates; resistanceheating means comprising a plurality of vertically extending lineartungsten filaments situated substantially equidistant from saidsubstrates, each of said filaments being prestressed to curve in asingle plane substantially equidistant from and parallel to saidsubstrates and fixedly secured at each end to a pair of fixedelectrodes; and a pair of moveable heat sinks situated adjacent saidsubstrates on the opposite side from said filaments, said heat sinkscomprising metallic copper with provision for passage of cooling water.